DMD modulated continuous wave light source for xerographic printer

ABSTRACT

An illumination system (10) for exposing a xerographic printing apparatus (12). The system (10) includes a DMD-type imaging spatial light modulator (46), and a DMD-type optical switch (26) for modulating the intensity of the source light (15) irradiating the imaging DMD (46). A single conventional continuous wave tungsten lamp (14) is implemented with its light energy directed by a condensing lens (20) onto the DMD optical switch (26). The DMD optical switch (26) modulates the incident light (15), and passes reflected light to a light integrator (38), which in turn homogenizes and increases the aspect ratio of the light. The light integrator (38) directs the homogenized light via an anamorphic lens (40) onto the imaging DMD (46). The light energy provided to the imaging DMD (46) is precisely modulated in intensity, while remaining uniformly disbursed. The combination incandescent lamp (14) and optical DMD switch (26) offers a low cost, high-intensity alternative to LED arrays.

CROSS REFERENCE TO RELATED APPLICATIONS

Cross reference is made to the following patent co-pending applications,the teachings of which are incorporated herein by reference:

    ______________________________________                                        SERIAL #    NAME             FILED                                            ______________________________________                                        08/221,739  Illumination Control Unit for                                                                   3-31-94                                                     Display System with Spatial                                                   Light Modulator                                                   07/809,996  System and Method for                                                                          12-18-91                                                     Achieving Gray Scale Spatial                                                  Light Modulator Operation                                         08/038,398  Process and Architecture for                                                                   03-29-93                                                     Digital Micromirror Printer                                       08/038,391  Gray Scale Printing Using                                                                      03-29-93                                                     Spatial Light Modulators                                          ______________________________________                                    

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to an image display system such as axerographic printer, and more particularly, to an illumination systemproviding high-intensity modulated light which facilitates gray scaleprinting.

BACKGROUND OF THE INVENTION

Semiconductor spatial light modulators (SLM's) are one viable solutionto realizing high quality, affordable xerographic printers. Onepromising SLM technology suitable for both printers and displays is thedigital micromirror device (DMD) manufactured by Texas InstrumentsIncorporated of Dallas Tex. The DMD is a monolithic semiconductor devicehaving a linear or area array of bi-stable movable micromirrorsfabricated over an array of corresponding addressing memory cells. Oneembodiment of a xerographic printer implementing a tungsten light sourcefocused via optics on an imaging DMD mirror array is disclosed in U.S.Pat. No. 5,041,851 to Nelson, entitled "Spatial Light Modulator Printerand Method of Operation", assigned to the same assignee as the presentapplication and the teachings included herein by reference.

In a xerographic printer implementing an imaging DMD spatial lightmodulator, it is desired to uniformly illuminate the DMD mirror arraywith a homogeneous light source such that each pixel mirror of the arraymodulates a uniform intensity portion of light. This is necessarybecause the DMD mirror array modulates this light to expose a lightsensitive rotating organic printing drum, whereby the intensity andduration of the modulated light directed thereon determines the relativeexposure of the charged drum. The exposed portion of the drum comprisesa latent image, wherein a quantity of toner will adhere to the drumimage, this toner then being transferred to a printing medium such aspaper, and fused thereon using heat.

It is also necessary that the energy of the light directed upon the DMDmirror array be of sufficient flux per unit area to fully expose therotating printing drum to obtain a dark image. If insufficient lightenergy is modulated and directed to the drum by the DMD mirror array,the printing drum may not be fully exposed, thus degrading the contrastof the image printed on a printing medium.

U.S. Pat. No. 5,159,485 to Nelson, entitled "System and Method forUniformity of Illumination for Tungsten Light", assigned to the sameassignee of the present invention and the teachings included herein byreference, discloses an anamorphic optical path arranged such that thevertical component of the source light is compressed to match thephysical shape of the DMD mirror array. The embodiment discloseddramatically increases the optical efficiency of the system, wherebylight energy is compressed to irradiate the DMD mirror array moreintensely from a given light source, such as a tungsten lamp.

U.S. Pat. No. 5,151,718 to Nelson, entitled "System and Method for SolidState Illumination for DMD Devices", also assigned to the same assigneeof the present invention and the teachings included herein by reference,discloses an array of LED emitters constructed to efficiently replacethe conventional tungsten source lamp. The LED array is geometricallyconfigured, and can be electrically operated by strobing to vary thebrightness of light to individual mirror pixels to achieve gray scaleimaging, and reduce fuzzy line images. Each of the LED's in the arraycan be provided with a lens to help columnate the light through opticsand onto the DMD mirror array. Using LED's, light is efficientlydirected and focused onto the DMD mirror array, with little light beingwasted and directed elsewhere. Less optical energy is required of thelight source compared to a conventional tungsten lamp to illuminate theDMD mirror array with a particular light intensity. The LED's can bequickly turned on and off, thereby providing the ability to modulate thelight energy directed upon the DMD mirror array, and consequently, helpsachieve gray scale printing. For instance, during a given line printcycle, the LED can be on for 50% of the cycle time to irradiate the DMDarray with half the light energy available for that particular timeinterval. However hi-power arrays of multiple LED emitters arerelatively expensive compared to conventional tungsten lamps. Moreover,the best state of the art LED devices suitable for electro-photographyare known to only reliably produce 200 milliwatts each. Accordingly, thealignment of the optics is critical to ensure that the energy of eachLED is directed upon the DMD mirror array. That is to say, the LED arraymay not produce sufficient and uniform light energy should one LED fail.

U.S. Pat. No. 5,105,207 to Nelson, entitled "System and Method forAchieving Gray Scale DMD Operation", assigned to the same assignee asthe present invention and the teachings incorporated herein byreference, discloses a system for enhancing resolution of a xerographicprocess by submodulation of each individual pixel. The submodulation isachieved by anamorphically reducing the square pixel presentation oflight rays to a rectangle having a number of controllable segmentswithin each square pixel scanned line. A conventional tungsten lamp isincorporated in this embodiment.

It is desirable to provide a low cost, high intensity optical systemwhereby the DMD mirror array can be uniformly illuminated with highintensity light. Moreover, it is desirable to provide a high intensitylight source which can be modulated in intensity to effect gray scaleprinting. Providing a low cost, single light source is preferred. Theoptical system should be easy to align, whereby any degradation in thelight source would be uniformly presented to the DMD array, and wouldnot noticeably degrade the printing quality of the xerographic printer.

SUMMARY OF THE INVENTION

The present invention achieves technical advantages as an illuminationsystem by implementing a DMD as a reflective optical switch modulating acontinuous wave light source. The DMD optical switch comprises thousandsof tiny micromirrors, each modulating incident light and directing themodulated light into a light integrator. The modulated light directedinto the light integrator is homogenized, and uniformly illuminates asecond imaging DMD spatial light modulator.

In the preferred embodiment of the present invention, an illuminationsystem for exposing a xerographic printing apparatus is provided. Thisillumination system comprises a spatial light modulator receiving andmodulating an incoming variable flux density light beam to expose thexerographic printing apparatus. A light source provides a continuouswave light, and an optical switch converts the incoming continuous wavelight to the variable flux density light beam, and passes the variableflux density light beam to the spatial light modulator.

Preferably, the optical switch comprises a reflective-type opticalswitch comprising a DMD. The spatial light modulator is offset at least40 degrees from the continuous wave light source with respect to theoptical switch, thereby keeping the optical elements from interferingwith the modulated light beam. In addition, the illumination system hasa short front-to-back depth. The light source preferably comprises aincandescent lamp, and the imaging spatial light modulator preferablycomprises a second DMD.

The illumination system further comprises a light integrator positionedbetween the optical switch and the spatial light modulator homogenizingthe variable flux density light beam. The light integrator uniformlyilluminates the imaging DMD device, and preferably is triangular inshape. A condensing lens is positioned between the continuous wave lightsource and the optical switch for concentrating the source light ontothe optical switch DMD device. A toroidal lens is positioned between thelight integrator and the spatial light modulator, although this devicecould be replaced with a reflective toroidal mirror to further reducethe front-to-back depth of the illumination system if desired. Aspectral filter is preferably positioned between the optical switch andthe spatial light modulator, preferably between the optical switch andthe light integrator. A control circuit controlls the optical switch,this circuit also controlling the imaging spatial light modulator.Because both the optical switch and the imaging spatial light modulatorare both DMD devices, the controlling circuit is suited to control bothdevices, simultaneously. This permits the imaging DMD to be controlledto achieve gray scale imaging, while also controlling the DMD opticalswitch to further modulate the source light and achieve gray scaleimaging. The DMD optical switch and the DMD spatial light modulator are,in a sense, complementary to each other to achieve gray scale imaging bycontrolling the on time of the mirrors.

The present invention also achieves technical advantages by providing anillumination system including a light integrator positioned between alight source and a spatial light modulator for homogenizing the lightsource and increasing the aspect ratio of the light. This preferablytriangular light integrator homogenizes the incoming light beam throughinternal reflection, and preferably illuminates a DMD spatial lightmodulator. This light integrator eliminates the need for focusing lensto direct incident light to a spatial light modulator, and has theadditional advantages of light homogenization, which is especiallycritical for uniformly illuminating a spatial light modulator, such as aDMD.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the illumination system according to thepreferred embodiment of the invention, including a DMD optical switchmodulating the intensity of a single continuous wave light sourceilluminating an imaging DMD via a light integrator to expose axerographic printing drum;

FIGS. 2A and 2B are diagrams illustrating the DMD as an optical switchwith particular mirrors in the on position to pass a proportionate anduniformly dispersed intensity of light to the imaging DMD for exposingthe xerographic photoreceptor drum;

FIG. 3 is a perspective view of one bistable DMD micromirror pixelcomprising both the DMD optical switch and the imaging DMD;

FIG. 4 is an optical schematic diagram of the light modulation achievedusing the DMD micromirror of FIG. 3 as used in the optical switch ofFIG. 1; and

FIG. 5 is a perspective view of an alternative preferred embodimentillustrating a light value being used to image the printing drum.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a perspective view of an opticalillumination system 10 for selectively exposing a rotating organicxerographic photoreceptor drum 12. Optical system 10 is seen to includea single high-intensity tungsten incandescent lamp 14 generating ahigh-intensity continuous wave light source 15, this light source 15being directed along optical path 16 for selectively exposing thexerographic drum 12. Optical system 10 modulates the intensity of thiscontinuous wave light, and also modulates discrete portions of the lightto expose and form a latent image on drum 12. This latent image of drum12 is then used to draw toner which is then transferred to print aprinting medium 18, such as paper, and fused thereon with heat.

As viewed from top to bottom, optical system 10 is shown to include anaspheric condensing lens 20 directing light 15 generated by a generallysquare lamp filament 22 onto an optical reflective-type switch 26.Optical switch 26 is preferably comprised of a DMD-type Spatial LightModulator (SLM), such as that manufactured by Texas Instruments ofDallas Tex. As shown, the intensity of continuous wave light 15 fromfilament 22 is directed at a central portion of a mirror array 28 shownin phantom at 30. That is to say, not all mirrors of mirror array 28will have incident light from lens 20. (See FIGS. 2A and 2B). As will bedescribed shortly, optical switch 26 uniformly modulates the intensityof light 15 directed thereon, and passes on by reflection the light to aspectral filter 32. Spectral filter 32 is well known in the art,filtering out light of unwanted wavelengths, and passing on light in theoptical spectrum of 600 to 800 nanometers in wavelength which iscompatible with common organic photoconductors. The wavelength of thelight which is passed by filter 32 is chosen to correspond with theideal optical wavelength properties for exposing photoreceptor drum 12.Thus, limitation to these wavelengths is not to be inferred, for it isonly preferred that the pass characteristics of filter 32 match theexposure characteristics of drum 12.

Light 15 is seen to be passed by filter 32, and focused by condensinglens 20 to a small proximal face 36 of a generally triangular lightintegrator 38. Light impinges upon face 36 normal to this surface, andis homogenized by scattering within integrator 38. The homogenized lightis uniformly directed to a typically refractive toroidal relay lens 40.Integrator 38 increases the aspect ratio of the incident light fromfront face 36 to a distal face 42. The distal (exit) face 42 ofintegrator 38 can be textured or polished to provide a lambertian lightdirector. Thus, the homogenized light is uniformly disbursed from lensface 42 to relay lens 40, as shown. Triangular integrator 36 has twosides 44 angled approximately 15 degrees from the central axis, asshown, to optimize the homogenizing and internal reflection of light ofselected wavelengths passed therethrough. Single light source 14 and asingle light integrator 36 efficiently homogenizes the light with lowcost associated optics. Lens 40 is elongated, and has a heightcommensurate with the height of distal lens face 42.

The homogenized light is passed by relay lens 40 and directed upon anelongated DMD type imaging spatial light modulator 46. The DMD SLM 46has a linear array of mirrors which modulate the converged homogeneouslight to form an image which exposes the photoreceptor drum 12, as iswell known in the art. For additional discussion of how DMD 46 modulatesan incoming light source to expose drum 12, cross reference is made toU.S. Pat. No. 5,101,236 to Nelson, et al, entitled "Light Energy ControlSystem and Method of Operation", assigned to Texas InstrumentsIncorporated, and the teachings of which are included herein byreference. For more detailed discussion of DMD's in general, crossreference is made to U.S. Pat. No. 5,096,279, entitled "Spatial LightModulator and Method" also assigned to Texas Instruments Incorporated,and the teachings of which are included herein by reference.

Basically, imaging DMD 46 is comprised of a linear array ofmicromirrors, each having a binary state to either reflect an incidentportion of light to or away from a projector lens 50. In the preferredembodiment, imaging DMD 46 is a linear array of micromirrors withdimensions of 64×7,048 mirrors, these mirrors being fabricated overaddressing circuitry including an array of memory cells and addressingelectrodes (not shown). Gray scale is achieved by modulating thesemirrors during a line printing frame, such as disclosed in U.S. Pat. No.5,105,207 to Nelson, entitled "System and Method for Achieving GrayScale DMD Operation", and U.S. Pat. No. 5,151,718 to Nelson, entitled"System and Method for Solid State Illumination for DMD Devices", bothof these patents being assigned to Texas Instruments Incorporated, andthe teachings of both patents included herein by reference.

The incident homogenized light is directed by imaging DMD 46 to theprojector lens 50, with the light image focused into lens 50 by lens 40.Projector lens 50 focuses the imaging light on xerographic photoreceptordrum 12. Both relay lens 40 and projector lens 50 ultimately focus thelight, whereby lens 40 is used to slightly overfill mirror array 48 witha uniform flux density light, and whereby projector lens 50 focuses thelight image generated by DMD 46 onto drum 12.

Referring now back to the front end of optical system 10, a moredetailed discussion of the unique light source arrangement will beprovided. First, referring to U.S. Pat. No. 5,151,718 to Nelson,entitled "System and Method for Solid State illumination for DMDDevices", it can be seen that one prior art arrangement for modulating alight source, which is then directed on and illuminates a DMD imagingarray, can comprise an array of LED's. These LED's can be modulated tocontrol the intensity of light passed to the imaging DMD array. Turningnow to the present invention as shown in FIG. 1, intensity modulation ofa light source is achieved by using DMD optical switch 26 in combinationwith a single conventional high-intensity tungsten lamp 14. Opticalswitch 26 is a reflective type switch, in contrast to a transmissivetype switch such as an LCD switch, whereby each micromirror 52 of array28 has an on or off position. In the on position, incident light isreflected by that particular mirror 52 along optical axis 16 to spectralfilter 32. In the off position, light is reflected along line 54 andpassed to a light collector 56.

Referring to FIGS. 2A and 2B, illustrative diagrams are presented toillustrate the portion 30 of mirror array 28 which is implemented tomodulate the intensity of incoming light 15. As shown, an array of 17micron square mirrors 52 are each individually deflectable between an onand off position, to either pass incident light to or away from lightintegrator 38, respectively. While only a few micromirrors 52 are shownfor purposes of illustration, array 28 preferably comprises an array of600 by 600 mirrors (360,000 total), with about 300,000 mirrors fallingwithin the generally elliptically shaped illuminated portion 30. Asshown in FIG. 2A, 75% of the mirrors 52 are shown in the on position as"white" pixels. These on mirrors 52 in turn reflect the portion ofincident light along optical path 16 to filter 32 and light integrator36. The off mirrors 52 are the "dark pixels", these mirrors reflectingthe respective incident light along line 54 to the light collector 56.With 75% of these mirrors in the on position, as shown in FIG. 2A ,75%of the incident light to array portion 30 will be reflected alongoptical axis 16 to filter 32 and light integrator 36.

Similarly, as shown in FIG. 2B, with 50% of the mirrors 52 in the onposition, 50% of the incident light to array portion 30 is reflectedalong optical axis 16 to filter 32 and light integrator 36. Likewise,50% of the incident light is reflected by the mirrors in the offposition along axis 54 and directed to the light collector 56. As shownin FIGS. 2A and 2B, the mirrors in the on positioned are selected to beuniformly dispersed to pass, as a whole, a balanced light beam.

Referring to FIG. 3 and FIG. 4, a basic DMD mirror 52 structure, andbistable operation for light modulation thereof, respectively, is shown.Mirror 52 is supported by a pair of collinear hinges 54 and supportposts 56 above an addressing substrate 58. Substrate 58 supports acorresponding pair of addressing electrodes 60 driven by an SRAM cell(not shown), and mirror tip landing pads 62. Switching controlelectronics 64 provides voltages to electrodes 60 via the SRAM cell, asshown in the cross referenced patents, to electrostatically deflectmirror 52. Referring to FIG. 4, each bistable mirror 52 can rotateplus/minus 10 degrees (θ) between an on and off position, eitherdirecting incident light (15) from collector lens 20 to or away fromfilter 32 and light integrator 38, respectively.

As shown in FIG. 1, lamp 14 is laterally positioned from integrator 38,with lamp 14 and condensing lens 20 shown to focus light from element 22at a 60 degree angle (6θ) with respect filter 32 and front face 36 oflight integrator 38. That is to say, when the mirrors 52 of array 28 arein the on position, light is reflected along path 16 at a 60 degreeangle from condensing lens 20 to filter 32 and light integrator 36.Since each of these mirrors 52 have a plus or minus 10 degree swing froma flat position, as shown in FIG. 4, for a total of 20 degrees rotation,light is reflected back along optical axis 54 at 20 degrees from theincident light when the mirrors are in the off position. Thisarrangement is particularly useful in spacing lamp 14 and lens 20 awayfrom the optical elements including filter 32 and integrator 38, andachieving a densely packed arrangement for packaging. In addition thecenter axis of integrator 38 is angled 14 degrees from a line normal tothe face of imaging DMD 46, both in the horizontal and verticaldirection, so as to not interfere with the imaged light directed by DMD46 into the focusing projector lens 50.

By implementing an optical DMD switch 26 at the front end of the opticalsystem 10, the intensity of light ultimately illuminating DMD imagingarray 48 can be precisely modulated to further enhance gray scaleprinting. As shown in FIG. 1, switching control electronics 64 isprovided to control DMD imaging array 46 as is well known in the art anddiscussed in the cross referenced patents, and which control electronics64 also control the switching (deflection) of mirrors 52 of opticalswitch 26 to modulate the incident light from lamp source 14 such asshown in FIGS. 2A and 2B. Electronics 64 preferably comprises rowaddressing circuitry and column pixel data loading shift registers.Electronics 64 simultaneously controls optical switch 26 and DMD array46 individually, but as a function of one another. Accordingly, the DMDarray 46 modulates the varying flux density light beam from DMD array26. In particular, the on time of the mirrors in array 46 is controlledduring a line print frame to control the gray scale of the pixel imagegenerated by each mirror, as discussed in the several cross referencedpatents. In addition, the on time of selected mirrors 52 of switch 26 isalso controlled to control the intensity or flux density of light beinguniformly projected upon and illuminating DMD switch 46. Because opticalswitch 26 is a reflective type switch, there is little loss in thisdevice, and a majority of the light intensity will be transmitted tofilter 32 and light integrator 36 when all mirrors 52 are in the onposition. In contrast, implementing a transmissive type optical switch,such as an LCD switch, would generate significantly higher losses evenwhen this switch would be in the fully on position. In addition, an LCDrise and fall time will be longer than the DMD.

As shown in FIGS. 2A and 2B, because array 28 is comprised of tinymicromirrors 52, each mirror typically being 17 microns square, theintensity of light transmitted to filter 32 can be preciselyestablished. In the preferred embodiment, array portion 30 is generallyelliptical, as shown, with a horizontal major axis of about one-halfinch in diameter, and includes about 300,000 mirrors. In the case of theDMD, even if some of these mirrors 52 are defective, even with say 1,000being defective, this still only represents about 0.3% of the usablemirrors 52 of array portion 30. Thus, a few damaged pixel mirrors willnot noticeably effect the accuracy of the overall intensity of themodulated incident light, and can be compensated for by simply not usingthe known defective mirrors.

Referring now to FIG. 5, an alternative preferred embodiment to thepresent invention is shown as optical illumination system 70. Opticalsystem 70 is very similar to optical system 10 as previously discussedand shown in FIG. 1. However, a light valve 72, such as a LCD switchmatrix, is implemented in place of DMD imaging array 46. Light valve 72is of the transmissive type, rather than the reflective type as the caseof the DMD imaging array 46. The remaining portions of this embodimentare identical to that of FIG. 1, and are discussed previously. Thehomogenized light passed to light valve 72 is selectively controlled byswitching control electronics 64 to create an imaged light, this imagedlight being focused by projector lens 50 onto photoreceptor drum 12. Asshown in FIGS. 1 and 5, the present invention is not limited to theparticular type of imaging array which controls the imaging and exposureof photoreceptor drum 12.

The present invention finds technical advantages as a low cost, highintensity, homogeneous light source arrangement. A single high-intensitycontinuous wave tungsten lamp 14 is implemented, together with anaffordable high resolution and high speed optical switch 26 to provide amodulated intensity light source illuminating the imaging DMD 46. Thecondensing lens directs the high intensity light on the mirror arrayportion 30 of switch 26, and focuses the light on the light integrator36, whereby this lens, switch, and optical integrator are inexpensiveand easy to optically align. Optical switch 26 is optically efficientsince it is a reflective type switch and generates little opticallosses.

Moreover, the DMD switch 26 has thousands of mirrors which can beswitched on and off in as little as one nanosecond, thus lending to highswitching speeds to achieve precise modulation of the incident light.Light can be reflected by each mirror for only a fractional portion ofthe print line window, thus helping to reduce the smearing effect whichtends to be produced when exposing a rotating receptor drum. As shown inFIGS. 2A and 2B, selected mirrors can be turned on to precisely effectthe overall percentage of light that is ultimately reflected to imagingarray 46. By evenly distributing which mirrors are on and off, a uniformand balanced light pattern is reflected to light integrator 36. As shownin FIG. 2B, even when 50% of the mirrors are in the off position toreflect only 50% of the light impinging array portion 30, still, auniform distribution of light is reflected by the distribution of onmirrors.

The single lamp 14 produces an abundance of light energy, with enoughlight being captured by condensing lens 20 for adequately irradiatingDMD imaging array 46. In contrast, the High Output LED array as shown inthe prior art, is expensive, with the best available LED's producingonly 200 milliwatts. While an LED can switch on and off fast enough,utilizing an array of LED, such as a linear array of thirty two LED's,is very expensive because of the electronics required to drive andadjust each LED to achieve uniformity. By implementing an affordable DMDoptical switch, the individual mirrors can be quickly turned on and offto modulate the incident continuous wave light.

In the embodiments shown, a conventional 50 watt tungsten lamp 14 incombination with an aspheric condensing lens 20, DMD optical switch 26and light integrator 36 can be provided for less than $50.00. This isconsiderably less expensive than the LED array of the prior art, whichmay cost hundreds of dollars. In addition, one malfunctioning LED in anLED array may be noticeable in the optical system. The loss of severalmirrors, even hundreds of mirrors, would not noticeably degrade theuniformity or intensity of light ultimately impinging imaging array 46.Mirrors which are known to be bad can be ignored by control electronics64, whereby only the functional mirrors can be implemented to modulatethe incident light. Finally, the implementation of a single light sourcefocused on the light integrator provides an efficient light homogenizerfor the evenly illuminating an imaging device, such as the DMD imagingarray 46.

In summary, a single conventional continuous wave incandescent light incombination with a robust DMD-type optical switch is utilized to providea low cost, fast, high intensity modulated light source for illuminatinga DMD-type imaging array. A low cost, typically plastic, lightintegrator is used to collect and homogenize the modulated light fromthe DMD optical switch and uniformly irradiate the DMD imaging array 46.With 300,000 mirrors residing within array portion 30, the resolution ofthe variable intensity light directed to filter. 32 and light integrator36 can be precisely controlled. By controlling the distribution of theon and off pixels, as shown in FIGS. 2A and 2B, the uniformity of thelight passed to filter 32 and integrator 36 is also controlled. Sincethe optical DMD switch 26 is similar in design to the DMD imaging array46, the switching control electronics is well suited to control both ofthese devices simultaneously, and as a function of one another toachieve gray scale printing. Thus, additional complicated and expensivecontrol electronics is not necessary. The DMD switch is a reflectivetype switch, rather than a transmissive type switch, and thus canreflect a height intensity light with little loss. The lamp source andcondensing lens are angled 60 degrees from the spectral filter and lightintegrator, and thus are advantageously spaced from one another fortight packaging without interfering with one another.

Though the invention has been described with respect to a specificpreferred embodiment, many variations and modifications will becomeapparent to those skilled in the art upon reading the presentapplication. It is therefore the intention that the appended claims beinterpreted as broadly as possible in view of the prior art to includeall such variations and modifications. For instance, refractive toroidallens 40 could be substituted with a reflective toroidal mirror to reducethe front-to-back depth of system 10, and possibly reduce sphericalaberrations, or be removed all together although this is not desired. Inaddition, the light source 15 could be directed by condensing lens 20 tooverfill the DMD array 28 with light, with mirror portion 30 modulatingthe associated incident light.

I claim:
 1. An illumination system for exposing a xerographic printingapparatus, comprising:a) a spatial light modulator receiving andmodulating an incoming varying flux density light beam to expose saidxerographic printing apparatus; b) a light source providing a continuouswave light; and c) an optical switch converting incoming said continuouswave light to said varying flux density light beam, and passing saidvarying flux density light beam to said spatial light modulator.
 2. Theillumination system as specified in claim 1 wherein said optical switchconsists of a reflective-type optical switch directing said varying fluxdensity light beam to said spatial light modulator.
 3. The illuminationsystem as specified in claim 2 wherein said spatial light modulator isoffset at least 40 degrees from said continuous wave light source withrespect to said optical switch.
 4. The illumination system as specifiedin claim 1 wherein said optical switch comprises a first DMD.
 5. Theillumination system as specified in claim 1 wherein said light sourcecomprises an incandescent lamp.
 6. The illumination system as specifiedin claim 1 wherein said spatial light modulator comprises a second DMD.7. The illumination system as specified in claim 1 further comprising alight integrator positioned between said optical switch and said spatiallight modulator and homogenizing said varying flux density light beam.8. The illumination system as specified in claim 1 further comprising acondensing lens positioned between said continuous wave light source andsaid optical switch.
 9. The illumination system as specified in claim 7further comprising a toroidal lens positioned between said lightintegrator and said spatial light modulator.
 10. The illumination systemas specified in claim 1 further comprising a spectral filter positionedbetween said optical switch and said spatial light modulator.
 11. Theillumination system as specified in claim 8 wherein said condensing lensis aspherical.
 12. The illumination system as specified in claim 9wherein said toroidal lens is aspherical.
 13. The illumination system asspecified in claim 1 further comprising circuit means for coordinatingcontrol of said optical switch with said spatial light modulator.
 14. Amethod of exposing a xerographic printing apparatus, comprising thesteps of:a) generating a continuous wave light source; b) modulatingsaid continuous wave light source with an optical switch to provide avarying flux density light beam; c) modulating said varying flux densitylight beam with a spatial light modulator to generate an image; and d)exposing said xerographic printing apparatus with said image.
 15. Themethod as specified in claim 14 including the step of utilizing a DMD asan optical switch to modulate said continuous wave light source.
 16. Themethod as specified in claim 14 including the step of utilizing a DMD asa spatial light modulator to modulate said varying flux density lightbeam.
 17. The method as specified in claim 14 further comprising thestep of coordinating said step (b) with said step (c) to controllablyexpose said xerographic printing apparatus.